Method for handling hard metal

ABSTRACT

A method for sorting hard metal including the steps of subjecting one or more bodies of hard metal to a heat treatment at a temperature of 500° C. or higher in an atmosphere including at least one reactive gas component prone to form a reaction product with an element that may or may not be present in the one or more bodies. A sorting operation of the one or more bodies is performed based on a presence or absence of the reaction product in a surface portion of the heat treated one or more bodies.

RELATED APPLICATION DATA

This application is a §371 National Stage Application of PCTInternational Application No. PCT/EP2012/061279 filed Jun. 14, 2012claiming priority of EP Application No. 11170382.3 filed Jun. 17, 2011.

The present invention relates to a method for sorting hard metal.

BACKGROUND OF THE INVENTION

Hard metal is widely used in a broad application range including cuttingtools, mining tools and wear parts.

Hard metal products, such as cemented carbides and cermets, containrelatively expensive elements, and there is a strong need to recoverscrap by recycling spent and unused hard metal products, both forenvironmental and cost reasons. There are several processes currentlyused to recycle hard metal products including, e.g., the zinc processand the cold stream process.

However, the hard metal often contains elements that make the recycledmaterial unsuitable as raw material base for the manufacture of certainhard metal grades. Such elements may be for example titanium, tantalumand chromium. These elements are commonly used as property enhancingadditions in a variety of cemented carbide grades. These elements are,however, for other grades unwanted or even detrimental to theproperties. There exist methods for purifying the raw material, mostlyof chemical nature, but these are time-consuming and costly, and ofteninvolve environmentally unfriendly process steps or chemicals.

EP 233 162 discloses a method for separating cemented carbide bodiesmaking use of the driving force for re-distribution of a binder metalmelt between hard metal bodies, in contact with or close to each other,having different mean grain size, grain size distribution, relativeproportions and compositions of the hard constituent phases. There ishowever still a need to sort hard metal bodies with regards to thepresence or absence of different elements in the hard metal.

THE INVENTION

It is an object of the present invention to reduce or eliminate theabove mentioned problem. This object is achieved according to theinvention by a method for sorting hard metal according to claim 1.

A method for sorting hard metal according the present inventioncomprises the steps:

(a) subjecting one or more bodies comprising hard metal to a heattreatment, at a temperature of 500° C. or higher, in an atmospherecomprising at least one reactive gas component prone to form a reactionproduct with an element that may or may not be present in the one ormore bodies, and(b) performing a sorting operation of the one or more bodies based on apresence or absence of the reaction product in a surface portion of theheat treated one or more bodies.

DETAILED EMBODIMENTS OF THE INVENTION

It has been found that the presence of a specific element in a sinteredhard metal body can be detected by subjecting the sintered hard metalbody to a suitable gaseous atmosphere at an elevated temperature. Underthe appropriate conditions, the presence of the element in the bodyresults in a reaction product being formed, wherein the reaction productcomprises the specific element comprised in the hard metal and anelement from the gaseous atmosphere. The reaction product may alsoinclude further elements from the hard metal and/or from the gaseousatmosphere. The formed reaction product, formed in the surface portionof the hard metal, affects the hard metal such that a subsequent sortingoperation can be based on the character of the affected part of the hardmetal. The absence of any formed reaction product during the treatmentat elevated temperature may in a similar manor be used for determiningthe absence of a specific element in the hard metal and thus allowingfor a subsequent sorting operation of such a body or bodies.

In this context a surface portion may not only include the outermost,visible surface, but can extend to a depth of 50 μm of more. Theaffected part may have a thickness of less than 1 μm up to several tenthof μm. Suitably the thickness of the affected part is at least 0.5 μm,but less than 50 μm. The affected part may further not necessarilyinclude the outermost surface, but may instead be entirely located belowthe outermost surface, i.e., in a non-visible part of the surfaceportion. For reasons of detection and characterisation of the formedreaction product it is preferred that the surface portion extends to adepth of no more than 100 μm and thus the affected part is suitably atleast partly located within this portion.

The present invention thus relates to a method of sorting hard metalcharacterised by comprising the steps:

(a) subjecting one or more bodies comprising sintered hard metal to aheat treatment in an atmosphere comprising at least one reactive gascomponent prone to form a reaction product with an element that may ormay not be present in the one or more bodies, at a temperature of 500°C. or higher, and subsequently(b) performing a sorting operation based on the character or appearanceof a surface portion of the one or more bodies.

In one embodiment the at least one reactive gas component comprisesoxygen. The oxygen can be added in the form of e.g. O₂, H₂O, CO, and/orCO₂. A reactive gas component comprising oxygen may suitably be used fordetecting e.g. Cr in the hard metal, the reaction product thus having acomposition comprising chromium and oxygen.

In another embodiment the at least one reactive gas component comprisesnitrogen suitably added as nitrogen gas, N₂, or ammonia. The use of areactive gas component comprising nitrogen is useful for detecting e.g.Ti in the hard metal, the reaction product thus comprising titanium andnitrogen.

In the above embodiments reactive gas components comprising oxygen andnitrogen are mentioned as suitable reactive gas components. However,other reactive gas components may be used which form detectable reactionproducts with one or more elements in the hard metal. Other elements inthe hard metal that may suitably form a reaction product are e.g. Ta, Nbor Mo. Tests have shown that a reaction product in sufficient amount forsubsequent sorting form even if the element is present in smallquantities in the hard metal. For instance, under the appropriate heattreatment conditions it is possible to form a sufficient amount ofreaction product even if the quantity of the element in the hard metalis as small as 0.4 weight-%.

The temperature used in the heat treating step is chosen to give areaction product of sufficient amount within an acceptable time, whilstat the same time avoiding possible detrimental effects on the hardmetal. Suitably the temperature is at least 500° C. In order to increasethe reaction rate the temperature is alternatively at least 900° C. Inyet another alternative the temperature is at least 1200° C., preferablyat least 1300° C. The heat treatment is suitably performed at atemperature of less than 2000° C. In order to avoid unwanted reactions,such as e.g. carburization reactions, and/or for economical reasons theheat treatment temperature is suitably less than 1800° C., preferablyless than 1700° C.

As is the case regarding the temperature, the time is also chosen toachieve a sufficient amount of reaction product without causing anegative effect on the hard metal. The time is suitably conducted for atime of at least 30 minutes. The time may however be as long as 16hours, for instance if a low temperature must be chosen to avoidunwanted reaction products being formed or to avoid excessive graingrowth to occur. In other cases grain growth is not a problem or mayeven be preferable, thus allowing for long heat treatment times incombination with a high temperature.

Another means for adapting the heat treatment is by choosing anappropriate partial pressure of the reactive gas or gases. Theappropriate partial pressures of the reactive gas or gases are suitablydetermined using thermodynamic calculations as know in the art. Apartial pressure of the at least one reactive gas component in the gasmay suitably be 10 mbar or higher. The pressure may however besubstantially higher, generally allowing for a shorter heat treatmenttime. However, in general a partial pressure of the reactive gascomponent of about atmospheric pressure or less is sufficient to form areaction product within a reasonable time. A lower partial pressure ofthe reactive gas component may be chosen to avoid unwanted reactions,which may be desirable if several different (specific) elements arepresent in the hard metal. The lower pressure may thus be sufficient toachieve a reaction with one of these elements in the hard metal, but atthe same time too low to result in a reaction with another of theseelements. Choosing an appropriate temperature may have a similar effect.

Thus, by using e.g. thermodynamic calculations the heat treatment may besuitably designed, with respect to e.g. the pressure of the one or morereactive gases as well as any inert gas present in the atmosphere andthe temperature, to detect specific elements in the hard metal, ifpresent, to allow for a subsequent sorting operation.

Temperature, time and gas pressure(s) may thus be chosen within broadranges. It is within the purview of the skilled artisan to choose anappropriate set of parameters and to verify that the desired result isachieved.

The method is generally applied on used or unused hard metal scrap thusthe one or more bodies are, before the heat treating step (a), suitablyheated from a temperature of below 200° C. up to the heat treatmenttemperature.

The method according to the invention is suitably applied on hard metalwhich is WC-based, i.e. the main hard constituents being tungstencarbide in a binder of an iron group metal, Co, Ni and/or Fe, suitablymainly Co. The hard metal may additionally contain other hardconstituents such as carbides or nitrides or mixtures thereof comprisingone or more of Ti, Ta and Nb. Other alloying elements may also bepresent in the hard metal such as e.g. Cr, Mo and/or Mn.

The method is suitably applied on hard metal bodies wherein the elementoptionally present in the one or more bodies is chromium. Underappropriate heat treatment conditions the presence of chromium in thehard metal results in a detectable reaction product. For example thereactive gas component may be oxygen thus forming a reaction productcomprising chromium and oxygen. The reaction may for instance be formedas an outermost part of the surface portion, typically appearing as ablackish surface colour, useful for visually inspecting the heat treatedhard metal body or bodies and sorting of the same. A suitabletemperature range for the heat treatment is between 950 and 1100° C. andthe time suitably less than 10 hours. The partial pressure of oxygencontaining gas, e.g. CO, CO₂, etc, in the furnace atmosphere may e.g. bechosen from the range 10 to 800 mbar. It is within the purview of theskilled artisan to determine a suitable combination of time, temperatureand partial pressure of the at least one reactive gas component and anyother gases present in the atmosphere for the heat treatment step, forexample by simple trial tests with reference bodies of know compositionor by thermodynamic calculations.

The method is suitably also used for sorting hard metal bodies whereinthe element optionally present in the one or more bodies is titanium.Under appropriate heat treatment conditions the presence of titanium inthe hard metal results in a detectable reaction product. For example thereactive gas component may be nitrogen thus forming a reaction productcomprising titanium and nitrogen. The reaction may for instance beformed as an outermost part of the surface portion, typically appearingas a yellowish surface colour, useful for visually inspecting the heattreated hard metal body or bodies and sorting of the same. A suitabletemperature range for the heat treatment is between 1300 and 1700° C.and the time is suitably less than 4 hours. The partial pressure of thenitrogen containing gas, e.g. ammonia, N₂, etc, in the furnaceatmosphere may in one alternative be chosen from the range 100 to 1100mbar. As mentioned above it is, however, within the purview of theskilled artisan to determine a suitable combination of time, temperatureand partial pressure of the at least one reactive gas component for theheat treatment step, for example by simple trial tests with referencebodies of know composition or by thermodynamic calculations.

Thus, in one embodiment of the invention the reaction product at leastpartly constitutes the outermost surface of the heat treated one or morebodies that contains the element.

In one embodiment the sorting operation (b) is based on a colour of theoutermost surface of the one or more bodies. In this context it is anadvantage if the colour is detectable by the naked eye allowing for amanual sorting operation or detectable by e.g. a visualisation systemallowing for automatic sorting.

In one embodiment, the sorting operation (b) is performed by visualinspection of the character of an outermost surface portion of the oneor more bodies.

In another embodiment, the sorting operation (b) is performed byautomated analysis of the character of the surface portion of the one ormore bodies.

In one embodiment, the hard metal body or bodies are uncoated prior tothe heat treatment.

In one embodiment, the hard metal body or bodies have a coating prior tothe heat treatment. The coating may e.g. be a PACVD, CVD or PVD coating.Tests have shown that such a coating does not obstruct the formation ofa reaction product. Instead a coating of e.g., TiN appear to initiallydissolve during the heat treatment where after a reaction product may beformed e.g. as an outermost surface of the hard metal.

In the above mainly a sorting step sorting based on a reaction productof a distinguishable colour forming the outermost surface of the hardmetal has been discussed. However, as an alternative, the sorting stepmay be based on the composition or other specific properties of thereaction product, suitably using an automated sorting system.

It should be noted that the same gas atmosphere may give rise todifferent reaction products for different hard metal compositions,giving a possibility to sort a group of hard metal bodies of severalhard metal grades, processed in the same single heat treatment step,into several different categories.

The method is suitable for sorting hard metal bodies of various types,sizes and shapes, for example inserts, such as inserts for cuttingmetal, wood, stone, etc., drills, and wear parts.

Example 1

Partly coated and uncoated samples of two different sintered hard metalcompositions were subjected to a treatment according to the invention.

The hard metal compositions were according to Table 1, other elements onlevel of impurity. Before the heat treatment the hard metal of all thesamples have a mid-grey colour.

TABLE 1 Sample Composition Coating a 10 Co, 0.5 Cr, balance WC(Ti_(0.8)Al_(0.2))N b 10 Co, 0.5 Cr, balance WC — c 11 Co, 4 Nb, 1.5 Ta,3 Ti; balance WC (Al_(0.7)Cr_(0.3))N d 11 Co, 4 Nb, 1.5 Ta, 3 Ti;balance WC —

The samples a-d were subjected to a heat treatment, wherein:

Temperature: 1650° C.

Time (at 1650° C.): 60 minutes

Furnace atmosphere: atmospheric pressure, nitrogen atmosphere of

-   -   N₂ (flow 500 l/h)        The results after the heat treatment can be seen in Table 2.

TABLE 2 Appearance of hard Sample metal surface Comment a Silver colourNo traces of coating residues b Silver colour c Yellowish colour Notraces of coating residues d Yellowish colour

From Table 2 it can be concluded that the samples are clearlydistinguishable from each other making a subsequent sorting operationeasy using, e.g., only the naked eye. Furthermore, the process iseffective in removing the hard and wear resistant coating initiallypresent on the hard metal, which is a great advantage in subsequentrecycling steps such as for example a Zn-process.

Example 2

Partly coated samples of the same two hard metal compositions as inExample 1 were subjected to a treatment at different temperaturesaccording to the invention.

Before the heat treatment the hard metal of all the samples have amid-grey colour.

TABLE 3 Sample Composition Coating e 10 Co, 0.5 Cr, balance WC(Ti_(0.8)Al_(0.2))N f 11 Co, 4 Nb, 1.5 Ta, 3 Ti; balance WC(Al_(0.7)Cr_(0.3))N

The samples e and f were subjected to a heat treatment, wherein:

Temperature: 1350° C., 1400° C., 1450° C., 1500° C., 1550° C.,respectively

Time (at top temperature): 60 minutes

Furnace atmosphere: atmospheric pressure, nitrogen atmosphere of

-   -   N₂ (flow 300 l/h)        The results after the heat treatment can be seen in Table 4.

TABLE 4 Temperature, Appearance of hard Sample ° C. metal surfaceComment e 1350 Mid-grey colour Coating residues e 1400 Mid-grey colourSome traces of coating residues e 1450 Mid-grey colour Minor traces ofcoating residues e 1500 Silver colour No traces of coating residues e1550 Silver colour No traces of coating residues f 1350 Yellowish colourNo traces of coating residues f 1400 Yellowish colour No traces ofcoating residues f 1450 Yellowish colour No traces of coating residues f1500 Yellowish colour No traces of coating residues f 1550 Yellowishcolour No traces of coating residues

From Table 4 it can be concluded that the Samples are clearlydistinguishable from each other when using a heat treatment temperatureas low as 1350° C. making a subsequent sorting operation easy. Theexample indicates that even lower temperatures may be used. Furthermore,the process is effective in removing the hard and wear resistant coatinginitially present on the hard metal.

Example 3

Uncoated samples of different sintered hard metal compositions weresubjected to a treatment according to the invention.

The hard metal compositions were according to Table 5, other elements onlevel of impurity. Before the heat treatment the hard metal of all thesamples have a mid-grey colour.

TABLE 5 Sample Composition (wt-%) g 10 Co, 0.5 Cr, balance WC h 13.5 Co,1.5 Cr, balance WC i 11 Co, 4 Nb, 1.5 Ta, 3 Ti; balance WC j 8 Co,balance WC

The samples e and f were subjected to a heat treatment, wherein:

Temperature: 1000° C.

Time (at top temperature): 8 h

Furnace atmosphere: atmospheric pressure, oxygen containing atmosphereof

-   -   H₂ (flow 0.6 l/h),    -   CO₂ (flow 0.066 l/h),    -   CO (flow 1.9 l/h)        The results after the heat treatment can be seen in Table 6.

TABLE 6 Appearance of Sample hard metal surface Comment g Dark greycolour h Near black colour i Mid-grey colour No apparent colour changeafter heat treatment j Mid-grey colour No apparent colour change afterheat treatment

From Table 6 it can be concluded that the chromium containing samplesare clearly distinguishable from both the samples of pure WC—Co as wellas samples containing Ta, Nb and Ti in addition to WC—Co, making asubsequent sorting operation easy.

The invention claimed is:
 1. A method for sorting hard metal comprisingthe steps of: subjecting one or more bodies of hard metal to a heattreatment at a temperature of 500° C. or higher in an atmosphereincluding at least one reactive gas component; forming a reactionproduct wherein the reaction product comprises specific elements foundin the one or more bodies and the at least one reactive gas component;and performing a sorting operation based on the character of theaffected part of the one or more bodies the sorting based on a presenceor absence of the reaction product in a surface portion of the heattreated one or more bodies.
 2. The method according to claim 1, whereinthe at least one reactive gas component is oxygen.
 3. The methodaccording to claim 1, wherein the at least one reactive gas component isnitrogen.
 4. The method according to claim 1, wherein a partial pressureof the at least one reactive gas component in the gas is 10 mbar orhigher.
 5. The method according to claim 1, wherein the temperature isat least 900° C.
 6. The method according to claim 1, wherein thetemperature is less than 2000° C.
 7. The method according to claim 1,wherein the heat treatment step is conducted for a time of at least 30minutes.
 8. The method according claim 1, wherein the hard metal isWC-based.
 9. The method according to claim 1, wherein said one or morebodies are heated from a temperature of below 200° C. up to the heattreatment temperature prior to the heat treatment step.
 10. The methodaccording to claim 1, wherein said element present in the one or morebodies is chromium.
 11. The method according to claim 1, wherein saidelement present in the one or more bodies is titanium.
 12. The methodaccording to claim 1, wherein said reaction product at least partlyconstitutes the surface portion of the heat treated one or more bodiescontaining said element.
 13. The method according to claim 1, whereinsaid sorting operation is performed by visual inspection of an outermostsurface of the one or more bodies.
 14. The method according to claim 13,wherein said sorting operation is based on a color of the outermostsurface of the one or more bodies.
 15. The method according to claim 1,wherein said sorting operation is performed by automated analysis of thesurface portion of the one or more bodies.